The present invention relates generally to the production of can ends and, more particularly, to a control assembly for detecting abnormal can end press operating conditions associated with the production of defective can ends and for providing an appropriate system response to the abnormal operating condition.
The production of modern beverage cans includes the production of can bodies, the production of separate can ends, and the subsequent attachment of can ends to the can bodies after the can bodies have been filled with beverage. The production of can ends generally includes the separate production operations of can end blank formation and can end conversion. During can end blank formation, sheet metal is cut and formed into generally circular-shaped can end blanks. Each blank includes a generally flat central region and a curved, upwardly extending, peripheral flange region. During the subsequent operation of can end conversion, the can end blanks pass through a can end press having a plurality of conversion die stations at which different work operations are performed on the can end blank. Typical operations performed on a can end during movement thereof through a can end press include the scoring of the flat region of the can end to provide a weakened region which will subsequently form an opening in the can end after rupture thereof by a consumer; the formation of a rivet adjacent to the scored region; attachment of a tab to the rivet to facilitate the rupturing of the scored region; the embossing of the flat region of the can end to provide predetermined alphanumeric characters on the surface of the can end; and the debossing of a predetermined area in the flat region of the can end to take up metal slack in the can end. Can end presses for performing the above-described operations have long been used in the art. U.S. Pat. No. 4,608,843 of Grims, issued Sept. 2, 1986, for CONVERSION DIE WITH DOUBLE END SENSOR describes the general operation of a prior art can end press and is hereby specifically incorporated by reference for all that it discloses. U.S. patent application Ser. No. 219,046 of Castor et al. for CAN END TAB SENSING APPARATUS, filed Jul. 14, 1988, now U.S. Pat. No. 4,939,665, describes an end press testing station and is hereby specifically incorporated by reference for all that it discloses.
A typical prior art can end press includes a stationary press frame having a lower bed portion and a upper crown portion positioned in spaced relationship and fixedly attached to one another at four corner portions thereof by four stationary frame post members. A fixed lower die is attached to the press bed and a reciprocally movable upper die is attached to the press crown. During a press operating stroke, the upper die is moved toward the lower die portion. A plurality of operating stations are provided with upper tool portions of each operating station mounted on the upper die and lower tool portions mounted on the lower die. During a press operating stroke, an upper die tool portion is moved into contacting engagement with a can end mounted on a lower tool portion of the associated press operating station. The operating stations are arranged in linear alignment, and can ends are moved from one operating station to the next between die press strokes by a transfer assembly.
A continuing problem in can end production has been the occurrence of abnormal press operating conditions which cause can end defects. Typical defects include: excessive scoring of the metal which produces can end leaks; improper rivet formation which may result in can end leaks or improper tab attachment; improper debossing which may result in printed matter on the can end being of insufficient depth to be readable or of excessive depth and causing rupture of the can end; etc. Typical causes of abnormal press operation include: metal scrap in one or more of the press operating stations; tool wear in one or more of the press operating stations; wear in press stop blocks (which are typically provided on a press to precisely limit the movement of the upper die portion during a press operating stroke); changes in metal blank thickness or hardness; etc.
Prior to the present invention, it was known to mount strain gauges on the four stationary press frame posts for the purpose of detecting gross press overloading. However, prior to the present invention, no method existed for determining the existence of abnormal operating conditions associated with the production of defective can ends other than random monitoring of can ends subsequent to end press conversion for product defects. A problem with such random monitoring when an abnormal operating condition exists for only a relatively few number of press strokes is that the defective can ends produced during the short duration abnormal operation may not be among those which are actually examined. Another problem with random defect monitoring is that many of the can end defects are difficult to detect even when a defective end is examined. Another problem with random monitoring is that, even when a defective can end is detected, it is generally unclear, from the mere discovery of the defective end, exactly when the abnormal operating condition which caused the can defect occurred. Thus it is often necessary to dispose of many can ends which are not defective or to conduct time-consuming and expensive examination of many batches of can ends in order to ensure that defective can ends produced during the abnormal operating condition are not used in subsequent production steps. Yet another problem with existing can end quality control operations is that the cause of an abnormal press operating condition, and thus the method for correcting the abnormal operating condition, may not be readily apparent to a press operator from the nature of a discovered can end defect.